Atomic number 72 · Transition metal
Hafnium Hf
Hafnium (Hf), element 72, is a hard, grey, corrosion-resistant transition metal. It is chemically almost a twin of zirconium and very hard to separate from it, yet the two behave in opposite ways towards neutrons: hafnium is used in reactor control rods, and hafnium oxide has become a key insulating layer in modern transistors.
Key properties
| Atomic number | 72 |
|---|---|
| Atomic weight | 178.49 (CIAAW 2024 abridged standard atomic weight) |
| Category | Transition metal |
| Group | 4 |
| Period | 6 |
| Block | d |
| State (25 °C, 1 atm) | Solid |
| Melting point | 2506 K (2232.8 °C) |
| Boiling point | 4876 K (4602.9 °C) |
| Density | 13.3 g/cm³ |
| Electronegativity (Pauling) | 1.3 |
| Atomic radius (van der Waals) | 212 pm |
| First ionization energy | 6.825 eV |
| Electron affinity | — |
| Oxidation states | +4 |
| Discovered | 1923 |
Electron configuration
- Condensed
- [Xe]6s2 4f14 5d2
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d2 6s2
- Electrons per shell
- 2 · 8 · 18 · 32 · 10 · 2
Orbital diagram
Position in the table
Discovery
Hafnium is a textbook case of theory guiding discovery. Some chemists expected element 72 to be another rare earth, but Niels Bohr's model of the atom predicted that it would sit in group 4 and resemble zirconium.
In 1923, at Bohr's institute in Copenhagen, Dirk Coster from the Netherlands and George de Hevesy from Hungary followed this prediction, examined zirconium ores by X-ray spectroscopy and found the characteristic lines of element 72. Georges Urbain had earlier claimed element 72 as "celtium", but his claim did not hold up. Hafnium was the second-to-last element with stable isotopes to be discovered; rhenium followed in 1925.
Origin of the name
The name comes from Hafnia, the Latin name of Copenhagen, the city where it was found.
Main uses
- Reactor control rods: hafnium absorbs thermal neutrons well and resists corrosion in hot water, making it a favoured control-rod material, especially in naval reactors.
- Zirconium purification: zirconium for fuel cladding must be almost transparent to neutrons, so hafnium is stripped out of it; most nuclear-grade hafnium is a by-product of this step.
- Semiconductors: hafnium oxide has a high dielectric constant and, from around 2007, replaced silicon dioxide as the gate insulator in leading-edge transistors. Ferroelectric hafnium oxide is being explored for new memory types.
- Alloys and tools: it strengthens nickel superalloys and serves as the electrode tip in plasma cutters.
Isotopes
Natural hafnium has six isotopes, with hafnium-180 the most abundant at about 35%; hafnium-174 is radioactive with an extremely long half-life. The extinct radionuclide hafnium-182 (half-life about 8.9 million years) decays to tungsten-182, and the hafnium-tungsten system is used to estimate how quickly the cores of Earth and asteroids formed. The nuclear isomer hafnium-178m2, with a half-life of about 31 years, stores a large amount of energy and was the subject of controversial research.
In everyday life
The processor in your phone or laptop probably contains hafnium oxide layers only a few atoms thick. Bulk hafnium is stable, but fine powder can ignite in air. Its toxicity is generally considered low.
Good to know
- Hafnium and zirconium have almost identical atomic radii, so they are always found together in nature.
- In nuclear engineering zirconium lets neutrons through while hafnium absorbs them.
- Tantalum hafnium carbide has been studied as one of the highest-melting materials known.
Same group (4)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.